In metropolitan areas it is not uncommon to run communications cable in underground ducts which are located adjacent to steam lines. The operating condition of the steam varies generally from 0.7 MPa (100 psig) to 2.8 MPa (400 psig). The corresponding minimum temperature of the steam, which is higher if superheated, varies from 165.degree. C. (328.degree. F.) to 230.degree. C. (445.degree. F.).
The rupture of underground steam piping is of great concern to telecommunication operating companies. Optical fiber cables placed in the neighborhood of steam pipes are vulnerable to damage and failure when there is a steam leak or when a pipe ruptures. When a high pressure steam pipe ruptures, adjacent optical fiber cable is exposed to a high temperature, high moisture, and high velocity environment. The values, of course, depend on the distance between the location of the rupture in the steam pipe and the cable. It is estimated from field data that the maximum steam temperature to which the cables may be exposed when there is a steam pipe leak or rupture could be as high as 140.degree. C. (284.degree. F.). The duration of exposure could be as long as a few months. Currently available commercial optical fiber cables have been found to fail in such an environment. The failures are not surprising because cables presently made were not designed for such an environment. The high temperature, high moisture, and high velocity environment degrades or melts the cable materials, resulting in the failure of optical fibers over a period of time.
Because steam may have an adverse affect on the communications cable, it is incumbent upon cable manufacturers to provide a cable having a sheath system which is capable of preventing damage to optical fiber when the cable is in service adjacent to steam piping. The primary considerations for a steam-resistant optical fiber cable are the high temperature and high moisture conditions. These conditions relate directly to the basic high temperature and hydrolytic stability performance of the cable materials. A sheath system of optical fiber cable suitable for use adjacent to steam lines must be such as to meet the high temperature and high humidity conditions. The high velocity condition is more closely related to the structural integrity and mechanical performance of the cable, and should be used as a secondary design consideration of the cable.
In the past, polyethlene-jacketed, lead-shielded cables were used in steam environments. Not only was this arrangement very costly, but the outer layer of polyethylene, when exposed to high temperatures for a long period of time, tended to melt or to develop cracks. Cables having a polyethylene jacket extruded over a soldered seam steel shield also have been used. However, in cables of this latter construction, the soldered seam generally has not been continuous.
In one steam-resistant, metallic conductor cable, a corrugated metallic layer surrounds a core and an extruded covering of polymeric materials surrounds the metallic layer. The extruded covering includes an inner jacket which is intimate contact with the metallic layer and which is interposed between the metallic layer and an outer jacket extruded over and in intimate contact with the inner jacket. The outer jacket is constructed of a material which is capable of withstanding exposure to temperatures of at least 100.degree. C. (212.degree. F.) and the material of the inner jacket has a notch sensitivity substantially less than that of the material of the outer jacket to maintain substantially the structural integrity of the cable notwithstanding corrugation imprint thereof by the metallic layer. Further, the inner jacket has a thickness which is at least slightly greater than the depth of the corrugations of the metallic layer.
What is needed and what seemingly does not appear to be available in the prior art is a steam-resistant optical fiber cable which is capable of providing reliable transmission notwithstanding exposure to a high temperature, high humidity and high velocity steam environment. Of course, such an optical fiber cable must not have an unduly large diameter and must be reasonable in cost.